Synthetic aperture radar, or SAR, has emerged as a valuable remote sensing tool. SAR is based on synthesizing an effectively long antenna array by electronic signal processing devices rather than by the use of a physically long antenna array. With SAR, it is possible to synthesize an antenna many times longer than any physically large antenna that could be conveniently transported. As a result, for an antenna of given physical dimensions, SAR has an effective antenna beam width that is many times narrower than the beam width that is attainable with a conventional radar.
An example of SAR is a pulse-Doppler SAR, which has a radiating element translated along a trajectory (e.g., mounted on an aircraft and transported along a predetermined flight path) to take up sequential sampling positions (e.g., locations along the synthetic antenna). At each of these sampling points, the radiating element transmits a radar signal (e.g., a radar pulse signal), and the pulse-Doppler SAR collects and stores returned signal data (i.e., the amplitude and the phase data of the radar signals received in response to a transmission). After the radiating element has traversed a distance substantially equivalent to the length of the synthetic array, the stored signal data contain information that can be manipulated to generate signal data that would have been received by the elements of a conventional linear array antenna (i.e., a non-SAR antenna).
The size of the unambiguous ground patch that can be imaged by the pulse-Doppler SAR is constrained in azimuth by the maximum frequency of repeating the pulses (also known as the maximum pulse repetition frequency, the “PRF”). In other words, the PRF should be high enough to sample unambiguously the maximum Doppler frequency within the azimuth mainbeam illumination pattern. Similarly, the range swath is constrained by the maximum pulse repetition interval (PRI), i.e., the time interval between pulses. The PRI should be large enough to preclude ambiguous returns from outside of the antenna mainbeam illumination pattern in the range direction.
Increasing the PRF allows for a wider unambiguous azimuth swath to be imaged; however, since PRI is the inverse of PRF, increasing the PRF would decrease the unambiguous range swath. Hence, while the area of the unambiguous patch on the ground is largely independent of the PRF/PRI selected, the length (range)-to-width (azimuth) ratio of the patch is directly affected by the selection of PRF. Since the antenna mainbeam is used to filter ambiguous returns in SAR systems, there is a direct relationship between the required physical minimum antenna area and shape, and the desired unambiguous ground patch area and shape. In general, the antenna area is set by the unambiguous patch size on the ground. However, a high PRF would require an antenna that is taller (in range) than it is wide (in azimuth), and vice versa for a low PRF.
In order to increase the maximum size of the unambiguous ground patch the collection platform could simultaneously operate, in effect, more than one radar receiver and/or transmitter. The Canadian Space Agency's Radarsat-2, for example, implements an approach for reducing the required receive antenna size (thereby improving the resolution, which is proportional to antenna width in strip mode) and maintaining an unambiguous collected swath in the Ultra-Fine mode by receiving simultaneously from two antenna phase-centers. However, the overall antenna size is not reduced due to the need for two displaced receive apertures. Hence, there is a need to resolve ambiguities beyond the traditionally recognized physical limitations described above, to thereby reduce the overall antenna size, weight, and complexity, or to increase the collected swath without increasing ambiguities.